Brachiopods are perhaps the most and, in some ways, least familiar of Ordovician fossils to the untutored eye. The most, because they are extremely abundant in sandstones, limestones and some shales, and everyone immediately feels a visceral recognition of their shells, so like the clams on the modern seashore. The least, because this understanding of them is utterly flawed.
The bivalved (‘two shelled’) clams and mussels are a group of molluscs (see section), and an extremely successful one at that. Today they are among the most prolific and diverse of seashore creatures, and their fossil record is extremely rich. At least, it is in relatively young rocks, especially from the Jurassic onwards. Go back into the Palaeozoic, and there are shells aplenty, equally diverse, and equally familiar. However, these shells were made by a fundamentally different creature; one that today still lurks in deep waters and shallow tropical seas, insubstantial and overlooked. They are still with us, but the brachiopods of today are just remnants of a glorious past.
Brachiopods are a group that probably share a close relationship with molluscs and with the annelid worms, but which have been evolving separately for at least 530 million years, since a time at which the ancestors of each group could not easily be recognised as belonging to them at all. The common ancestor may have been something rather like an armoured slug; our closest approximation is a remarkable Early Cambrian creature from northern Greenland, called Halkieria evangelista. However, the slug is a misleading impression to start with for brachiopods, for the soft tissues of the creature evolved to be smaller and smaller, retreating into its shells, until they were almost entirely enclosed. Then they were reduced even further, so that the inside of a brachiopod shell is mostly empty space, a smear of soft tissue with a muscular stalk, and a graceful whorl of filtering tentacles, the lophophore.
The soft tissues, however, are almost never preserved, even from exceptional faunas – they are simply too delicate, and decay too quickly. What we are left with are the shells, by the thousand. The skeleton consists not of left and right shells (as seen in bivalve molluscs), but of ventral (pedicle) and dorsal (brachial) valves. To confuse things further, they live upside down, so that the pedicle valve is actually on top. Each valve has a mirror-plane running through it from front to back (the animal is bilaterally symmetric, remember – think of the slug), but the two valves are different. They are divided into two fundamental groups, both of which are very important components of the Ordovician faunas, and we’ll consider them separately.
Inarticulate brachiopods.
The ‘inarticulates,’ or ‘lingulates,’ are rather unfamiliar-looking creatures, but their shells are extremely abundant in fine sediments, which they often inhabited in short, U-shaped burrows. The shape of the shell is rather conservative in general, and Lingula itself is often considered the ultimate ‘living fossil’ – it has retained a very similar form for the last 500 million years! – but some genera such as Monobolina and Opsiconidion show the range of form that is possible in the group. Their shells are made of organic matter with calcium phosphate, and usually appears brown or black when fresh. After being fossilized, it turns either blue-black or pinkish-white, with a variety of shiny greys also possible. Being resilient, though, the phosphate is rarely dissolved entirely, and fossils have a very different appearance from those of the calcareous ‘articulate’ brachiopods.
The inarticulates are so-called because their shells do not join at the hinge, but instead rely on a complex network of muscles to open and close the valves. Modern forms are often quite mobile, in some cases burrowing, and in others stuck to rocks, but occasionally swivelling to scrape their bristly setae over the surface, scouring it clean of other settlers. There is some evidence that some inarticulates nestled among Silurian corals, letting the colony grow around them, or occupying empty borings. Others were pseudoplanktic, attached to floating objects: seaweed, pumice or graptolites. In Ordovician black shales, they can be extraordinarily abundant, even when there is little other fauna, suggesting a tolerance for low-oxygen conditions. Oddly, though, individual recognisable species tend to be very widely distributed, and the total diversity of the Builth Inlier is only about seven species. Compared with the diversity and patchy distributions of articulate brachiopods, this is a strange thing indeed.
Articulate brachiopods.
The articulates are much more interesting to most collectors; they show a much greater range of shapes, are much more diverse, and have a sufficiently patchy distribution that one can always hope for another different one. In contrast to the inarticulates, they tend to be found in shallow-water, coarse sediments such as sandstones, and are usually associated with a diverse shelly fauna.
The shells are made of calcium carbonate (calcite, in this case), which in well-preserved material usually shows two structural layers. Mostly, though, they are preserved as moulds; the calcite is retained in limestones, but these are scarce in the Builth Inlier. Most shells have fine radiating ribs, and concentric growth lines on the outside. Although there are differences between them, it is only really on the inside that majority of differences become obvious, particularly among the diverse and abundant orthids. The inside of the shell is marked by muscle scars, plus a variety of hinge structures, and the patterns here can be used to distinguish between most species quite easily. This does mean that collectors should always look for the combination of different views – internal and external, pedicle and brachial – in order to get a complete idea of the species.
Besides the orthids, which are usually the smallest and least distinctive, there are strophomenids, which have broad shells in which one may rest inside the other, giving a concavo-convex profile. The rarer pentamerids have one or more plates inside the shell, effectively separating the interior into chambers. Even more uncommon are the occasional rhynchonellids, with sharply ridged shells and a zig-zag gape. Other articulate brachiopod groups, such as spiriferids and terebratulids, appear later in the fossil record.
Ecology - Decline and Fall.
The history of brachiopods is peculiar. They appeared, successfully, in the Cambrian, exploded into huge diversity during the Ordovician and Silurian, fluctuated through the rest of the Palaeozoic, until the devastating end-Permian extinction – and then never recovered. They have remained in relatively small numbers through the rest of history, although a brief renaissance in the Jurassic promised to bring them back to prominence. The reasons for this pattern are still unclear. There is a strong temptation to compare them with the bivalves, but this may not be as instructive as it appears, since their range of ecology is very different.
Bivalves are mobile, often burrowing, and have adapted to a huge range of niches. The articulate brachiopods, on the other hand, appear to have been entirely sessile, either fixed to some hard surface by a muscular pedicle, or resting on softer sediment. That accomplished, they sat where they were, filtered water, grew, reproduced, and did little else. They simply don’t have enough soft tissue to be capable of doing much more. While it is easy to make comparisons with such bivalves as mussels, it is not as if we see blankets of brachiopods over our rocky shores today. Something else has made them so much less successful than they were in the Palaeozoic, and what it is, is not obvious. One idea is that they were outcompeted by the bivalves, but this has not stood up to further testing. Another is that brachiopods are more likely to be eaten (for example, by starfish or boring snails) than bivalves. This only became a real problem after the end-Permian extinction, during the Mesozoic Marine Revolution, when predation, and anti-predator adaptations, became an escalating arms race.
Whatever the reason for the dominance of bivalves today, brachiopods appear to have dominated the Palaeozoic. However, this may not be the full story. Brachiopods, as mentioned earlier, have either phosphatic or calcitic shells. Bivalves have shells made of calcium carbonate, but a different form from articulate brachiopods. Bivalve shells (most of them, anyway) are made from aragonite, which is significantly less stable than calcite. This means that it is entirely possible for all the bivalve shells in an assemblage to be dissolved, but all the brachiopod shells to be preserved. Any passing palaeontologist will have no idea that there were bivalves there. How can we tell the difference between not finding any bivalves because there were never any bivalves living, and not finding bivalves because they were all dissolved? The answer is to look at silicified faunas, where all the calcium carbonate shells have been replaced by silica at a very early stage, before the aragonite has had a chance to dissolve. In Silurian and Carboniferous examples, there are significant numbers of bivalves in faunas that, when not silicified, contain no bivalves. So it appears that the apparent dominance of brachiopods in the Palaeozoic may be at least partly due to the fact that we’re simply not seeing the bivalves.
Brachiopods (‘articulate’)
[1]Brachiopod indet. A. Up to ~ 25 mm across.
[1]Brachiopod indet. B. Up to ~ 10 mm across.
[1]Brachiopod indet. C (Tissintia aff. prototypa?), with illustration of serial trifurcation rib branching pattern (on exterior). This is very similar to some specimens of Tissintia prototypa from Shelve, but differs from others in the narrowness of the 'teeth' on the brachial valve, and the restriction of costae to the margins on the interior surfaces. In most specimens attributed to T. prototypa, the costae are visible over almost the entire inner surface. It is likely that, in common with other species, what we define as a species is actually a much more variable group. What we need in order to establish whether these morphologies form a continuum are large enough collections of single populations (i.e. from one bed) to allow statistical studies. Up to ~ 15 mm across.
[2]Christiania elusa. Up to 15 mm.
[2]Corineorthis pustula. Up to 20 mm.
[2]Dalmanella parva. Up to ~ 15 mm.
[2]Glyptorthis viriosa. Up to 20 mm.
[2]Hesperorthis dynevorensis. Up to 25 mm.
[2]Horderleyella convexa. Up to 20 mm.
[2]Macrocoelia elongate. Up to 40 mm.
[2]Macrocoelia llandeiloensis. Up to 40 mm.
[2]Mcewanella berwynensis. Up to 15 mm.
[2]Parastrophinella parva. Up to 10 mm.
[2]Porambonites sp. Up to 35 mm.
[2]Rostriculella triangularis. Up to 12 mm.
[2]Salopia turgida. Up to 15 mm.
[2]Sowerbyella antiqua. Up to 30 mm.
[1,2]Tissintia plana. Up to 20 mm.
[1,2]Tissintia prototypa. Up to 10 mm.
[5]Tissintia sp. Up to 15 mm.
[1,2,3,4,5]Apatobolus? micula. The only fossil species known from all levels in the Builth Inlier, it is also extremely abundant and occurs in practically every environment. This includes black mudstones, with otherwise graptolitic faunas. There are rare examples of specimens clustered around 'Archiclimacograptus' sp., and this plus the ecological response to ash-fall indicates a partly pseudoplanktic habit. In other words, they attached themselves to anything, whether on the sea floor or floating at the surface. Up to 5 mm diameter, usually ~ 2 mm.
[2,3,4,5]Meristopacha granulata. Up to 25 mm.
[3,4,5]Monobolina ramsayi. Up to 30 mm.
[2,3,4]Opsiconidion nudum. Although very distinctive, it is known from only one locality each in the Builth Volcanic Group and the Upper murchisoni shales, the latter from only a single specimen and the former from three. It therefore appears to have quite specific environmental needs, and is an interesting species to take note of. Up to 5 mm.
[2,3,4,5]Palaeoglossa attenuata. Up to 10 mm.
[2,3,4]Paterula fissula. Up to 5 mm.
[2]Schizotreta cf. transversa. Up to 10 mm
.
[1] unidentified articulate(?) from the lowest graptolotic beds in the inlier
[1] inarticulate known only from fragments, with exterior(?) ornament of raised granules marking curved arcs (costae or 'growth lines')
[4] Shizocrania cf. salopiensis (Sheldon 1987; unpublished PhD thesis)
[4] "pitted brachiopod" (Sheldon 1987; unpublished PhD thesis)
[4] dalmanellid indet. (Sheldon 1987; unpublished PhD thesis)
[2] indet. patelliform brachiopod(?)
At least two species (presumed three at this stage) of poorly-known articulate(?) brachiopods from the Llanfawr Mudstone Formation, including one found encrusting coiled nautioloids.
